: Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by immune complex glomerulonephritis and infiltration of activated macrophages into the glomerulus. In the MRL/lpr mouse model of lupus, macrophages and mesangial cells are hyperresponsive to immune stimuli as evidenced by increased production of NO and TNF alpha compared to controls. Once activated by immune stimuli, macrophages and resident glomerular mesangial cells continue to secrete inflammatory mediators that maintain and enhance the local inflammatory state. Activators of the nuclear receptor PPAR gamma inhibit macrophage and mesangial cell activation. Our preliminary studies indicate that PPAR gamma expression is decreased in mesangial cells from MRL/lpr mice compared to controls. Furthermore, production of PG1J2, an intrinsic ligand for PPAR gamma, by MRL/lpr mesangial cells is significantly less than BALBIc mesangial cells. Our central hypothesis is that both at baseline and with immune stimulation there is a decrease in PPAR gamma expression and/or signaling in mesangial cells of MRL/lpr mice. Alterations in the PPAR gamma pathway result in a heightened and prolonged inflammatory response in MRL/lpr mice. To test this hypothesis the following specific aims are proposed: 1. Determine the levels of PPAR gamma protein, mRNA and regulation of its expression in mesangial cells obtained from MRL/lpr, MRL+/+, NZB/NZW, BALB/c, B6/lpr and B6 mice. 2. Determine the concentration-response dependent effects of PGJ2 and pioglitazone, a PPAR gamma agonist, on the synthesis and/or induction of NO/NOS2, TNF alpha, COX-1 and COX-2 and apoptosis, proliferation and matrix protein production in mesangial cells of MRL/Ipr, MRL+/+, NZB/NZW, BALB/c, B6/lpr and B6 mice stimulated with LPS/IFN gamma, IL-1 beta or TNF alpha. 3. Determine the time course of synthesis for PGJ2, PGD2, PGE2, PGI2, and TXA2 by mesangial cells from MRL/lpr, NZBINZW, BALB/c, B6/lpr and B6 mice and which COX is responsible for their synthesis. The effect of pioglitazone on their production will be determined. 4. Determine the effects of pioglitazone on the urinary excretion of NOx, protein, parent eicosanoids and metabolites, serum 3NT, GFR, renal, joint, and lung histology, and serum autoantibodies in MRL/lpr mice. To test these specific aims a combination of cell culture, molecular and cellular biology and in vivo animal studies will be performed. These studies will provide important insight into the role of PPAR gamma and its ligands in controlling the inflammatory response in lupus nephritis. We and others have shown that the inflammatory response in lupus nephritis can be blocked, decreasing proteinuria and pathologic renal disease, without affecting immune complex deposition. As agonists of PPAR gamma (the thiazolidinediones) are already approved for human use, insight gained from these studies can be quickly translated into human trials.